1. Field of the Invention
The present invention relates to an ac generator for a vehicle to be mounted in a vehicle or the like.
2. Description of the Related Art
An ac generator for a vehicle generates electric power by means of mechanical power that is supplied by an engine so as to charge a battery and supply electric power for engine ignition, illumination and operation of various electrical accessories. It is very important to make such an ac generator more compact, lighter in weight, more powerful and less expensive, in view of market competition.
Recently, various electric accessories have been adopted in order to comply with various laws and regulations and market demand for more sophisticated vehicles. Accordingly, power consumption of the vehicle has increased and heat generation by the ac generator increases, so that temperature of various parts becomes very high. Further, the increase in the various electrical accessories increases the degree of accumulation of the engine compartment, resulting in increasing the temperature of the engine compartment. Thus, it is very important to reduce the temperature of an ac generator.
In particular, temperature of the rectifying unit, which is disposed near other accessories, becomes higher than other accessories because it generates more heat as more current is supplied by the ac generator and because it is affected by heat radiation from other accessories. Therefore, it is the most important to reduce temperature of the rectifying unit.
Recently, a vehicle is equipped with various electronically controlled devices which includes an ECU (engine control unit). In such a vehicle, if a surge voltage is generated by the ac generator due to breakdown of a power line of the ac generator or abrupt shutdown of an electric load, the ECU or other devices may fail. Therefore, it is necessary to suppress such a surge voltage.
The following is disclosed in JP-A-4-244770 (pages 2–3, FIGS. 1–2) as one of conventional ways of cooling a rectifying unit: the negative electrode side heat sink of the rectifying unit is contacted with a generator housing and the positive electrode side heat sink is contacted with metal cover via a insulating member so that the rectifying unit can be cooled by way of heat transfer.
The following is also disclosed in French Laid Open Patent Application 2734426 (FIG. 1) as another way of cooling a rectifying unit: the negative electrode side rectifying element is not fixed to a heat sink but is directly force-fitted to a hole of the housing so as to directly transmit the heat generated by the rectifying element to the housing.
As disclosed in JP-A-64-8872 (Pages 2–3, FIGS. 1–2) , it is well known as a conventional art of suppressing surge voltage that all the rectifying elements are zener diodes. Further, there is a rectifying unit, as disclosed in JP-B2-4-24945 (page 2, FIGS. 1–3), in which the zener diodes are used in one group of the positive electrode side rectifying elements or the negative electrode side rectifying elements and normal diodes are used in the other group.
The rectifying unit of an ac generator for a vehicle disclosed in JP-A-4-244770 has an aim of cooling the rectifying unit by heat transfer from the heat sink to the housing or the metal cover. However, the temperature of the housing becomes very high because the housing encloses a stator core the temperature of which is the highest in the ac generator. Therefore, the heat transfer from the negative electrode side heat sink is not effectively carried out. In other words, a sufficient cooling effect can not be achieved.
The rectifying unit disclosed in French Laid Open Patent Application 2734426 has an aim of providing highly efficient heat transfer for better cooling effect by direct force-fit into the housing. However, a sufficient cooling effect can not be provided because the temperature of the housing becomes very high, as stated above. The positive electrode side heat sink does not particularly increase the cooling effect, resulting in insufficient cooling.
It is well known to use zener diodes for all the rectifying elements in order to suppress the surge voltage. In fact, the zener diodes have been widely utilized for that purpose. However, the zener diode is more expensive than the normal diode, resulting in increase in the cost of the ac generator. The rectifying unit disclosed in JP-A-64-8872 includes normal diodes for the positive electrode side rectifying element and zener diodes for the negative electrode side rectifying elements. In this rectifying unit, a high backward voltage may be applied to the normal diodes, so that a surge voltage may go out of an ac generator to an electronic device such as an ECU, which may be badly affected.
The rectifying unit disclosed in JP-B2-4-24945 has much more rectifying elements, and it is difficult to reduce the size and weight of an ac generator for a vehicle and the production cost thereof.